In general computer programs include a number of internal functions, plus computer codes that call these functions in a specific order. This approach works well when all of the necessary functions are available within a single program. However, there are times when a required function is located elsewhere. Such functions are normally referred to as remote, or external functions.
One way to make these remote or external functions available to a program is to incorporate them into the local program. When feasible, this is the most efficient approach. However, remote or external functions sometimes depend upon other things (e.g., data, operating systems, hardware, etc.), which may not be available to the local program. In such situations, importing the remote or external function to the local program is not possible. Hence, the only alternative is to invoke the desired function remotely. These are known as Remote Procedure Calls (RPC's), which are available for such use. RPC's operate much, much slower than internal functions, in fact they are four or more orders of magnitude slower.
Some systems provide a streamlined RPC mechanism for use in shared memory environments, which are referred to as Local Procedure Calls (LPC). This capability eliminates the overhead of moving a function call across a network and reduces the per call overhead to less than 1 microsecond with today's microprocessors. Local Procedure Calls, however, are only available when all of the functions are running under the control of one single operating system. In a heterogeneous multiprocessing (HMP) system, there is a desire to have two different operating systems closely cooperating to carry out certain tasks.
Technologies are available for carrying out this cooperation, which are variants of Remote Procedure Calls (RPCs). RPCs operate over a network transport of some sort, and serve to physically move a request from one environment to another. At best, they operate in the range of 100 microseconds overhead per call. While this overhead is acceptable for some long operations, it is excessive for short operations, making cooperation impractical. In addition, the 100 microsecond or greater overhead must be incurred by each function call, further reducing the desirability of RPCs. A function calling sequence with drastically reduced overhead is required.
In shared memory HMP environments, there is no need to physically move the function from the memory of one operating environment to the memory of the other. Both operating environments share a single memory. External Procedure Calls (EPC) take advantage of this fact to expedite the calling sequence between operating environments, allowing overheads of less than 1 microsecond using today's Instruction Processors (IPs). This overhead reduction allows EPCs to be used for cooperative processing where RPCs would be impractical.